1. The Field of the Invention
The present invention relates to glass or glass-ceramic plates, especially which provide cooking surfaces in cooking appliances, and their use in cooking appliances for oven doors, operating panels, and display regions.
2. Description of the Related Art
In cooking surfaces composed of glass or glass-ceramic, the configuration of regions in which information is imparted by means of illuminated displays (LEDs, LCDs etc.) is of increasing importance. Cooking surfaces made of colorless or slightly colored, transparent glass-ceramic, in particular, offer completely new opportunities compared to conventional cooking surfaces made of colored glass-ceramic.
Cooking surfaces composed of colorless glass-ceramic generally have a colored underside coating which blocks the view into the interior of the hob (of cables, metal supports, and the like). Simple leaving open of parts of the underside coating enables uncoated regions for displays (“display windows”) to be produced. Individual LEDs, 7-segment displays or entire LCD visual display units are arranged behind the small (a few millimeters) or large (e.g. 10 cm×30 cm) display regions of different shapes, so that it is possible not only to indicate the operating state of the cooking surface but also to retrieve and display, for example, cooking recipes by means of infrared or capacitive touch sensors.
However, depending on the design of the illuminated displays, it can be necessary to have a coating of the cook top panel in the display region too, e.g. to restrict the visibility of electronic components (circuit boards, etc.). Particularly high-quality coatings for display regions are known from, for example, EP 1 867 613 A1. The noble metal layers proposed there have a transmittance matched to the visible spectrum of light and therefore allow both the installation of red, white or otherwise colored LEDs and also the use of colored LCD visual display units. Furthermore, they barely scatter light, as a result of which the illuminated display is very bright and clear. However, disadvantages are the high manufacturing costs associated with the noble metal content of such layers and the restricted choice of colors (only black, brown, silver, gold or copper can be obtained).
However, less expensive display coatings which allow a variety of colors are also available on the market, although in this case restrictions in respect of the display quality are consciously accepted.
Thus, for example, FR 2 885 995 A1 and WO 2007/066030 A1 propose coatings based on binding agents such as silicones, polyimides or polyamides for cooking surfaces having display regions. The coating in the display region (“display coating”) is applied in areas left open in the first, color-imparting layer. Here, a color difference between the display layer and the surrounding, color-imparting layer is desired in order to emphasize the display region. The difference can be produced or reinforced by a different reflection of the display layer or a different pigmentation.
Disadvantages of the display coatings mentioned in FR 2 885 995 A1 and WO 2007/066030 A1 are the low scratch resistance and the greater scattering of the light from the display devices (as a result of the used pigments having particle sizes in the micron range). Testing of the scratch resistance of a commercially available cooking surface having a silicone-based display coating showed, that a rounded carbide metal tip (tip radius: 0.5 mm) which was adjusted perpendicular to the coating and was led horizontally across the display layer, even under a load of less than 100 g, gives a scratch trace which could be seen through the glass-ceramic, i.e. visible to the user. Therefore such display coatings require careful handling both during production and in subsequent assembly in order to keep low the costs incurred by scratched display layers (rejects). Furthermore, for aesthetic reasons the color difference between the display layer and the color-imparting layer is a disadvantage since cooking surfaces having a uniform color are desired at present.
According to JP 2003 086337 A, display coatings can also be produced by means of alkyl silicate paints (sol-gel paints). According to this Japanese document, the sol-gel-based display layer is applied as a first paint layer over the entire area of the underside of the cooking surface, and an opaque, black paint layer which is left open in the display region, is subsequently applied as light shielding layer. The display layer contains effect pigments and possibly other inorganic additives. As a result of the black light shielding layer, the effect pigments display their iridescent effect, so that the regions with the black coating have a different color from the display regions without the backing (the cooking surface is, for example, metallic grey in the region coated with two layers and white in the region coated with one layer).
The color difference between the display region and the other region can, according to JP 2003 297540 A, be reduced by providing the display region with dots of the black coating. Then the cooking surface has of least an approximately uniform shade of color. However, the display areas are only suitable for backlighting display devices, but unusable for, for example, 7-segment displays.
A significant disadvantage of the display coatings described in JP 2003 086337 A and JP 2003 297540 A is the dependence of the color of the surrounding area (hot region, cold region, etc.) on the color of the display coating. Since the display coating is applied as a first layer over the entire cooking surface, the color of the display layer continues to determine the color of the other regions of the cooking surface. The design possibilities for a cooking surface are considerably limited thereby.